TeachersDomain - http://www.teachersdomain.org/ (Subscription required to do more than 7 resources, but subscription is free and allows you to download and share.)
WorldClips.tv - http://www.worldclips.tv/ - This site has video clips, but they want you to pay for them. You can use the preview to show a concept. You just can't download and put into a PowerPoint without purchasing the license for the videos.
This is a limited list of photos, clipart, video, and music to use in student projects. Remember to follow Fair Use procedures and respect copyright!
Public Domain: belonging or being available to the public as a whole, and therefore not subject to copyright
Creative Commons: works available for others to build upon and share
Video Sources
Remember, video clips are powerful. A 30-minute video is passive. Three seconds to a minute or two might be enough.